1. Field of the Invention
The present invention relates to lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. Description of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. The mask table should be taken as any structure or device that may or does hold another structure or device, generally referred to as a mask, in which a pattern to be imaged is or can be formed. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire mask pattern onto the target area at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each target area is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination beam. To such end, it has been proposed to use wavelengths of less than about 200 nm, for example 193 nm, 157 nm or 126 nm. Further reductions in the wavelength of the illumination radiation, e.g to about 10 to 20 nm, are also envisaged. Such wavelengths in particular are more conveniently focused and controlled by reflective optics, such as mirrors. However, mirrors in lithography apparatus must be positioned to especially high accuracy, as compared to refractive elements, because any rotational orientation errors are magnified by the total downstream optical path length. In an apparatus using very short wavelength radiation, the optical path length may be of the order of 2 m or more.
For example, to have a good overlay performance, it can be necessary to keep the position of an image of the irradiated portion of the mask stable at a given position at substrate level with an error (e) of less than about 1 nm (see FIG. 3 of the accompanying drawings). If the distance between the mirror and the substrate (W) is 2 m the maximum permissible rotational error of the reflected beam, to keep the system within specification, is 28xc3x9710xe2x88x929 degrees (1xc3x9710xe2x88x929m/2m=tan 28xc3x9710xe2x88x929). Since, for a mirror, the angle of reflection equals the angle of incidence, a rotational error (da) in the position of the mirror will give rise to twice as large an error in the direction of the reflected beam. Thus the mirror must be positioned with an accuracy of 14xc3x9710xe2x88x929 degrees or better. If the mirror has a width of order 0.1 m and a rotating point at one side, that rotating point must be positioned to within 0.024 nm (tan 14xc3x9710xe2x88x929 xc3x970.1=2.4xc3x9710xe2x88x9211). Clearly the accuracy with which such a mirror must be orientated is extremely high and will only increase as the specification for image accuracy increases. The accuracy requirements for position in X, Y and Z are less demanding as such errors are magnified less at substrate level.
It is an object of the present invention to provide a lithographic projection apparatus having an improved positioning system to accurately and dynamically position a mirror in the radiation or projection systems.
According to a first aspect of the present invention, there is provided a lithographic projection apparatus, including:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table constructed and arranged to hold a mask;
a second object table constructed and arranged to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate,
wherein at least one of said illumination system and said projection system comprises one or more reflective optical elements and positioning means for dynamically controlling a position and/or orientation of one or more of said reflective optical elements.
The one or more reflective optical elements may comprise a single element such as a mirror, a reflective grating, a reflective filter, etc. or a combination of such elements with or without other types of element. With the invention, the position of the reflective optics is controlled continuously or repeatedly during operation of the apparatus and the effects of vibrations and mechanical shocks, and thermal and mechanical drift thereby can be mitigated.
Preferably, the projection apparatus further comprises sensing means constructed and arranged to determine a change in position and/or orientation of one or more of said reflective optical elements, and to output one or more position signals indicative thereof; and said positioning means comprises:
drive means constructed and arranged to change a position and/or orientation of one or more of said reflective optical elements in response to a drive control signal; and
a controller responsive to said one or more position signals for generating said drive control signal so as to correct for said determined change in position and/or orientation of one or more of said reflective optical elements.
In a preferred embodiment of the invention, the lithographic apparatus includes a reference frame and sensing means for determining the position of said reflective optics relative to said reference frame.
Also preferably, the sensing means includes:
an absolute position sensing means constructed and arranged to measure a position and/or orientation of one of said reflective optical elements and to output an absolute position signal indicative thereof; and
a relative position sensing means constructed and arranged to measure changes in said position and/or orientation of said one reflective optical element and to output a relative position signal indicative thereof.
Said drive means may be arranged to change said position and/or orientation of said one reflective optical element in response to said drive control signal; and said controller may be responsive to said absolute and relative position signals for generating said drive control signal so as to set and maintain said one reflective optical element in a desired position and/or orientation.
By the use of both absolute position sensing means, which can determine the absolute position and/or orientation of the reflective optics without calibration each time the apparatus is initialized, and relative position sensing means, which can detect movements in the position and/or orientation of the reflective optics with a high bandwidth and/or larger measuring range, the positioning system can accurately position, or stabilize, the reflective optics without a lengthy calibration or initialization procedure, and counteract any vibrations in the reflective optics. After an initial position determination using the absolute sensing means, the drive means are controlled primarily on the basis of the high frequency output from the relative sensing means or interferential encoders.
The absolute sensing means preferably include one or more capacitive or inductive sensors and the relative position sensing means preferably include one or more interferometers.
In yet another preferred embodiment said sensing means is constructed and arranged to direct a sensing beam of radiation separate from said projection beam along said one or more reflective optical elements; and to determine a position of said sensing beam when having been reflected by said one or more reflective optical elements.
According to yet a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table constructed and arranged to hold a mask;
a second object table constructed and arranged to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; the method comprising the steps of:
providing a mask bearing a pattern to said first object table;
providing a substrate provided with a radiation-sensitive layer to said second object table;
irradiating portions of the mask and imaging said irradiated portions of the mask onto said target portions of said substrate; and
dynamically controlling a position and/or orientation of one or more reflective optical elements comprised in one of said illumination and projection systems.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including, for example, ultraviolet radiation, EUV and X-rays. Also, the terms xe2x80x9cmirrorxe2x80x9d and xe2x80x9creflectorxe2x80x9d are used synonymously and, unless the context otherwise determines, are intended to encompass any reflective element, whether wholly, partially or selectively reflective and whether or not it has any other optical, e.g. refractive or diffractive, properties. Where the context allows, the term may also apply to non-specular reflectors such as scatter plates. The term position should be interpreted broadly as referring to any or all of the X, Y, and Z positions and rotational positions Rx, Ry and Rz.